The present invention relates to the field of particle synthesis, and more specifically that of the synthesis of particles having controlled dimensions, in particular nanometric dimensions (for example smaller than 500 nm, and in particular smaller than 100 nm), in particular smaller than 50 nm, or even 20 nm).
More specifically, the invention relates to a method for continuously preparing mineral particles, of the type implementing the thermolysis of the mineral precursors in an aqueous medium, and which can in particular be used for an industrial production.
The term “mineral precursor,” as it is used in this description, designates a chemical species, generally mineral or organometallic, preferably soluble or dispersible in an aqueous medium, which, when subjected to a thermal treatment in an aqueous medium, is converted into another chemical species, of a mineral nature, that is capable of forming solid mineral particles, through nucleation and growth, within the aqueous medium.
A number of precursors of this type are known, which most often lead, through thermolysis in an aqueous medium, to metal oxide-, metal oxyhydroxide- and/or metal-based particles. Examples of such mineral precursors are certain hydrosoluble metallic salts of transition metals or lanthanides (for example, non-limitingly, nitrates or sulfates, such as ZrO(NO3)2, Zn(NO3)2 or Fe2SO4, which are converted by thermolysis in an aqueous medium into zirconium, zinc or iron oxide-based particles, respectively). Aside from these metallic salts, various other precursors have been described, in particular organometallic compounds, such as for example bis(ammoniolacto)dihydroxytitanium (CH3CH(O)COONH4)2Ti(OH)2, which is converted into titanium oxide-based particles by means of thermolysis in an aqueous medium.
In certain cases, the precursors used can be species formed in situ, just before their conversion into particles, such as for example the hydroxides of transition metals or lanthanides (in particular iron or zinc hydroxides, obtained through the in situ reaction of a corresponding metallic salt (nitrate or sulfate, for example) and a Brønsted base (typically NaOH, KOH or NH4OH).
For mineral precursors of the aforementioned type, it is possible to define a threshold temperature, which will hereafter be referred to as the “conversion temperature,” beyond which the mineral precursors are converted into mineral particles. The conversion temperature of a given precursor generally varies with the pressure.
Advantageously, the conversion of the mineral precursors into mineral particles takes place in water in the supercritical state, i.e. a temperature above 374° C. and a pressure above 221 bar (22, 1.106 Pa), which makes it possible, among other advantages, to decrease the reaction time.
Different methods have been described to prepare particles by means of the thermolysis in an aqueous medium of mineral precursors of the aforementioned type, which often have the drawback of not leading to particles having homogenous sizes, compositions and morphologies.
In this context, methods have in particular been known for some time conducted by batches (i.e. in batch reactors). In these methods, most often, the reagents generally start to react as of the pressurization and heating up time of the reactor, which leads to a non-homogenous reaction and therefore disparities in the composition, size and/or morphology of the synthesized particles.
More recently, continuous implementation methods have been developed, in which a liquid flow comprising the mineral precursors is injected into a chamber brought to a temperature above the conversion temperature of the precursors. On this subject, one may in particular refer to the article by Arai et al. in J. Am. Ceram. Soc., vol. 75(4) pp 1019-1022 (1992), which describes a synthesis of this type making it possible to prepare metal oxide-based particles. The solution proposed by these methods conducted continuously certainly avoids a premature reaction of the reagents, but it is generally not sufficient to obtain completely homogenous synthesis conditions allowing the synthesis of particles with a homogenous composition, size and morphology. Among other difficulties, the particles obtained tend to pile up.
Alternatively, proposed by Lester et al. in the Journal of Supercritical Fluids, vol. 37, pp 209-214 (2006) is a particle synthesis where the thermolysis of the precursors is done by putting in contact, within a tubular reactor: (i) a first flow comprising mineral precursors; and (ii) a second, countercurrent flow comprising water at a temperature above the conversion temperature of the precursors. In this type of method, the injection of the flow of water raises the temperature of the medium conveyed in the tubular reactor, and, downstream of the injection point, the particles form through gradual conversion of the precursors. Here again, the synthesis conditions are generally not homogenous enough to allow the formation of particles with a homogenous composition, size and morphology. In particular, an entire zone downstream of the injection point exists where the temperature and composition of the reactive mixture are difficult to control.